Mirrors are made by the successive application of various metals, preferably silver and copper films of between 800 angstroms and 1,500 angstroms in thickness on suitable substrates of glass or plastics such as polycarbonates, by wet chemical plating processes or vacuum metallizing, followed by a paint coating application, usually known as a mirror backing.
It is known that reflective silver layers on mirrors, even when protected by "backing" coatings are extremely sensitive to corrosive decomposition when exposed to moisture, contaminants in the atmosphere, salt, hydrogen sulfide, ammonia, and chlorides, which are present in domestic environments. It is also known that zinc phosphate or zinc salts of dicarboxylic acids, commonly applied as corrosion inhibitors in protective primers on steel or aluminum, contrary to expectations, do not work well in mirror backing coatings and actually exhibit corrosive action on the silver layer.
It can be stated that in the presence of moisture, numerous contaminants of airborne origin or ionic species generated "in situ" by pigments of inadequate quality (more specifically by soluble salt-contaminants, which usually accompany pigments of inadequate quality), which are functional components of the protective coating system, promote corrosion processes on mirrors Ag (silver) layer. Consequently, it can be concluded that due to the above mentioned extreme sensitivity to the corrosive action of ionic species, a mirror Ag (silver) layer is quite intolerant to quality variations of raw materials, more specifically of pigment components employed in "Mirror Backing" protective coatings.
Such quality variations of pigment grade products, typically those obtained by wet processes, are generally caused by the presence of trace amounts of soluble inorganic contaminants, usually water soluble by-products of the manufacturing procedures. A reduced specific surface area (reduced porosity) of solids, usually grades produced by various calcination procedures, is generally accountable for the reduced level of reactivity. Such reduced reactivity of pigment grade products (notably grades of basic mono-zinc cyanamide produced by calcination procedures) should also be considered quality variations intolerable in mirror backing applications designed for long term corrosion preventive protection. It is reasonable to suppose that the above mentioned intolerance of coating protected Ag (silver) mirrors, to the quality variations (specifically to the presence of the above mentioned soluble salt contaminants) of commercially available pigment grade products, is the reason why certain pigment grade zinc cyanamides have not gained acceptance in particular applications as in mirror backings as lead free alternatives to widely employed lead based pigments, and more specifically of lead cyanamide.
Various mirror backing coatings have been proposed by the prior art, including coatings which comprise mixtures of an organic resin and pigment. For example, U.S. Pat. No. 4,707,405 to Evans et al, discloses a mirror backing coating which comprises an organic film forming resin and a cyanamide salt of a non-lead metal. The cyanamide salt is said to be a salt of group IIA or group IIB metal such as calcium, zinc or magnesium, with the preferred salt being calcium cyanamide. It is believed that the calcium and magnesium cyanamides have not been successful because both are water soluble, highly alkaline "non-pigment" products which interfere in detrimental fashion with curing processes of coating systems organic material. In addition, they exhibit only marginal corrosion preventive activity on silver, which is expected considering that typical technical grade calcium cyanamides contain considerable amounts (e.g., 1% CaS.sub.2) of soluble sulfide species as impurities. The zinc cyanamide mentioned above proved to be a basic mono cyanamide containing contaminants and inclusions of ZnO, ZnCO.sub.3 and SO which have prevented such cyanamides from performing satisfactorily under commercial conditions in mirror backings.
The present invention provides improved paint and coating formulations for mirror backings which provide excellent long term corrosion preventive protection of mirror reflective silver layer.